This discussion has become really interesting. I'm a complete interloper in this thread and I haven't even studied the Iapetus images properly but I was intuitively uneasy about the accumulating sandpile idea and just wanted to see how far we could get with an alternative. I hope some of the others join in too. Where for example is Richard Trigaux? I'm sure he would have something to say about this. In the meantime can I repeat Rob Pinnegar's request for a quick link to the best pictures showing multiple ring ridges and/or diverging chevrons - anybody got them handy?

I also cautiously and with all due respect note some of the test footage shown on NASA TV of ice impacts on wing structures during the Columbia accident investigation. Ice was fired at the test samples in a speed range not too far short of the possible touch down speeds of ring materials onto Iapetus. To my untrained eye, (even though the tests appeared to be conducted at room temp and not at -300 F) it appeared the ice did not appreciably wet the surfaces it contacted. Rather, it just tended to pulverize into 'snow'. (in fact, one could see the pulverization occured at the instant of contact, the speed of sound (and fracturing) in the ice being so much higher than the impact speed).

What you do not see, even in the high speed film clips, is how much ice is immediately vaporized - perhaps some of it quickly recrystalizing. It is a substantial amount (I will try to find a quantity). During these and similar test, quite heavy steal supporting brackets were deflected and bent.

I have to wonder if this is true. When F-16's collide with the desert at similar velocities, they expect the remains of the pilot to weight 18-25 lbs - if there is no cockpit fire. Virtually all of the liquids - water, uncontained oils and fuel - are immediately vaporized.

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Note, some gas/particle spray liberated at the point of contact will interact with the materials still orbiting above that point. Any material passing through that area of 'spray' will not complete another orbit of Iapetus and will land downrange along the ground track. This is why the main ridge (and the 2 attendants too) slope down away from the high end.

I like your analysis of the 'bulldozer effect', but I don't see water at any temperature as the source of this ridge deposit - if so, it should look more like drifting snow than Paul Bunyan and his plow.

but I don't see water at any temperature as the source of this ridge deposit - if so, it should look more like drifting snow

Why? Are you assuming that environmental parameters at the time and place of deposition were pretty much as they are there today? If so that's a pretty drastic assumption - and just a bit too convenient in my view - given that we are talking about rather a large mass of material falling from space over an unknown period of time. I imagine something a lot more chaotic and violent, involving a wide range of temperatures and matter in all three states. At the very end of the process, yes, there was probably a fine hail of ice particles falling through near-vacuum onto a deep-frozen surface, but I think things would have been a lot more messy as the bulk of the material was coming down.

In such a situation there is too wide a field of possibilities and too much room for contingency, for example in the pattern of sizes, collisions and perturbations among the larger ring fragments, for a 'tidy' explanation that claims to characterise the whole process from start to finish to be convincing, IMHO.

One thing everyone seems to like (myself included) is the idea that the Iapetan ridge is indeed the remains of a fallen ring. Until this discussion I had not realised the dynamical implications of the remoteness of Iapetus from Saturn - that it could sustain a ring, and perhaps previously a satellite, of its own.

When F-16's collide with the desert at similar velocities, they expect the remains of the pilot to weight 18-25 lbs - if there is no cockpit fire. Virtually all of the liquids - water, uncontained oils and fuel - are immediately vaporized.

a) Yuck.

Those liquids are already liquid. How are they "fleeing" the scene? It's not necessary that they be raised to the boiling point, is it? If a water balloon were struck by a bullet, much of the water would "depart" the vicinity, but I don't think any of it (or almost none) would actually boil.

Would ice have to be made gas to vanish from a microimpact on Iapetus, or merely liquid? That's a different phase transition, in the first place, and one that must take place, while the jet impact case may not require any at all. The heat of vaporization of water is much higher than the heat of fusion, but overall, it's a lot more energy to boil H2O that starts at saturnian temperatures than at body temperature.

Why? Are you assuming that environmental parameters at the time and place of deposition were pretty much as they are there today?

Yes, Aside from assuming that the belly band started as some kind of ring or dissintegrating moon structure.

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In such a situation there is too wide a field of possibilities and too much room for contingency, for example in the pattern of sizes, collisions and perturbations among the larger ring fragments, for a 'tidy' explanation that claims to characterise the whole process from start to finish to be convincing, IMHO.

There are as many potential pitfalls in working backward to find a cause, as there is to working forward to predict an effect. Cassini is doing everything it was designed to do, but we seem to be finding more questions than answers. That's not bad, but it leaves us two options: Make some assumptions and try to prove them wrong on based on the evidence that is here, or wait for another mission.

We need to find better solutions than 'dark stuff' and water ice. We need to narrow down the list of materials by taking a hard look at the physical properties and eliminating what does not fit. What defines the surface besides color? Why are the surfaces of Titan and Iapetus so different from Enceladus and Hyperion?

we seem to be finding more questions than answers. That's not bad, but it leaves us two options: Make some assumptions and try to prove them wrong on based on the evidence that is here, or wait for another mission.

There is another option: to try to sketch out the whole space of possibilities opened up by the observations rather than placing bets on a single winner prematurely. A future mission will have a lot more than one hypothesis to check out.

On the scale of entire worlds and their histories nature is profligate with ways and means, not economical. We cannot just proceed as we would in a laboratory experiment by looking for the single most 'elegant' or 'economical' explanation.

Hi,let us anticipate the equatorial ridge was built up by an ancient ring orbiting Iapetus.

How likely is it now for CASSINI to detect any remnants of this former ring structure?Or put it the other way - is it possible for a ring to vanish completely without leaving any dust particles in orbit?

Therefore a Iapetus ring model must be developed to show how such a ring is behaving over ages.Iapetus - as most distant large moon of any large planet - should be able to retain a ring for a while.

For mass assumption the ridge mass should be adopted.There is still the question whether the ring was built up by one major event, e.g. a large impact like the 'snowman' or the huge southern bassin, or did Iapetus act like a sort of 'vacuum cleaner' in Saturn's outer vicinityto collect all kind of interplanetary debris?

The ridge is ancient which means the ring is also ancient. Any material left in orbit would probably be perturbed and dispersed away over a long course of time. I wonder how solar light pressure would evolve the ring particles, gently pushing on them. The ring system was probably very unstable, given Iapetus' weak gravity and probably pretty significant perturbations by Saturn (It's conceivable that after the impact that created the rings, Iapetus' rotational axis wasn't perpendicular to the orbital plane as it is today so Saturn's perturbations might have played a role. This along with the increasing "bulginess" of Iapetus itself would make stable orbits pretty hard to achieve.) I wonder if the impactor would melt Iapetus completely. If so, would it wind up undifferentiated now as it's suggested?There's also the question of meteoroid bombardment on the rings and scattering them away. Of course, all this is arm-waving without any real numbers to back it up.

Personally, though, this ring idea seems too far-fetched for me.

BTW, Cassini did a pretty entensive search for moonlets during a recent fairly close approach. AFAK, no objects or ring remnants were seen.

Hi,let us anticipate the equatorial ridge was built up by an ancient ring orbiting Iapetus.

How likely is it now for CASSINI to detect any remnants of this former ring structure?

Highly unlikely. The ring episode must have been very early, and possibly quite brief in geological terms, as there has been much heavy bombardment since. I would be very surprised (though delighted!) to find anything still in orbit around Iapetus.

A comment on followup missions: Whatever future missions head to Titan, Enceladus, or Saturn are going to have to cross Iapetus's orbit once on the way in. In some cases, mission design would allow an opportunistic single pass by Iapetus, perhaps quite closely, while costing essentially nil in terms of the rest of the mission.

I have little doubt that a good optical survey of Iapetus's various longitudes would give us some definitive evidence regarding its mysteries. Cassini has one more look in store: It will be sufficiently close-up, but may (or may not) show us some of the less-interesting longitudes. If this one-look turns out to be uninformative, we will likely wait until some Enceladus/ring mission gives us one-look somewhere else. As has been mentioned elsewhere, it might be possible to get Cassini to perform another close encounter of Iapetus, but the cost might be rather extreme, and I doubt it will happen. We'll get the answer next year or not anytime soon.

A comment on followup missions: Whatever future missions head to Titan, Enceladus, or Saturn are going to have to cross Iapetus's orbit once on the way in. In some cases, mission design would allow an opportunistic single pass by Iapetus, perhaps quite closely, while costing essentially nil in terms of the rest of the mission.

True, but that intersection isn't likely to fit the arrival hyperbola so careful arrival timing (on the order of years!) is likely to be required.

I'm not an expert in orbital dynamics, but is Iapetus' orbit ascending node fixed w/ respect to the stars? A probe entering the Saturnian system at a hyperbolic trajectory will probably have a more-or-less fixed point (arrival angle w/ respect to the sun and with a given injection energy) where closest approach is made. Take a simplification: the arrival plane is Saturn's equatorial plane. The intersection, c/a possible points are then Iapetus' ascending and descending node. Then, you have to wait until the Saturn's revolution around the Sun rotates one of the nodes to the point where the approach trajectory interects the Iapetus' orbital radius. Only then is the arrival geometry right.This would constrain the possible arrival times to the Saturn system to two fairly short periods each Saturn orbit, each half an orbit apart. That'd be a long delay between launch windows.

This is the simplest case, but I hope you get the picture. Am I grossly in error here?

Not to me! In fact it seems obvious now, and I'm kicking myself for not thinking of it first. In the chaotic early days of the solar system it's quite likely that almost every globe that wasn't too close to something else would have hosted transient rings from time to time. That includes all the major planets and perhaps quite a few of the remoter moons, perhaps also KBOs. The question is which worlds would be likely to preserve the evidence? Clearly there must be no resufacing, no erosion, and the body must be very rigid. That makes Iapetus the best candidate among the worlds we've seen close up, (though it might be worth revisiting the available images of Callisto with this in mind). The Kuiper Belt is a whole new ball game however. With so many binaries and multiple systems out there ring formation must have occurred sometimes, and the resulting equatorial ridges should be well preserved. How fortunate that we'll get our first look in just a few years' time . . .

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